The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. H 11-30921.
The present invention relates to an electron gun capable of emitting an electron beam with high emittance suited for use in an electron beam exposure apparatus.
FIG. 4 schematically illustrates a standard structure adopted in electron beam lithography apparatuses. An electron gun 122 comprises a cathode 123, a Wehnelt 124 and an anode 126. The center of an electron beam EB emitted from the electron gun 122 is adjusted to align with the optical axis at an alignment coil 127 and the peripheral unfocused portion is cutoff at a first aperture 128. Then, the electron beam EB passes through a reducing lens system 129 where, the beam diameter becomes reduced, and is focused through an objective lens 131 onto a surface to be drawn 132. It is to be noted that although not shown, a deflecting electrode is provided between a second aperture 130 and the surface to be drawn 132.
A negative voltage of 10xcx9c100 kV is applied to the cathode 123 heated to a high temperature (e.g., 1200xc2x0 C.). The potential at the anode 126 is normally set at 0V, and an electron beam is emitted from the cathode 123 toward the anode 126. The Wehnelt 124 achieves a potential a slightly lower than that at the cathode 123, i.e., a voltage that is negative relative to the voltage at the cathode 123 is applied to the Wehnelt 124. As a result, an electric field, in which the electron beam traveling from the cathode 123 toward the anode 126 is set closer to the optical axis, is formed. The electron beam originating from the cathode 123 is accelerated as it advances toward the anode 126, and passes through a central hole formed at the center of the anode 126 to travel in the downward direction in the figure.
In such an electron gun used in an electron beam exposure apparatus adopting a probe forming method which is employed for drawing pattern, the electron emitting surface (area) at the front end of the cathode is pointed to achieve a high level of brightness.
In addition, cathodes having wide and flat electron emitting surface (e.g., a cathode having an electron emitting surface with a 0.2 mm diameter) have been developed for application in electron beam exposure apparatuses adopting the cell projection method in which projection exposure is implemented over a small area (e.g., a 5xc3x975 xcexcm area on a reticle), in order to achieve high emittance characteristics.
As a means for heating such a flat cathode (referred to as the main cathode), a sub-cathode is provided toward the rear surface of the main cathode to impart an electron bombardment from the sub-cathode to the rear surface of the main cathode in the known art. In order to prevent the electron beam originating from the main cathode from becoming bent, the sub-cathode is constituted by using a filament coiled to achieve minimum induction.
It is also known that a highly uniform temperature distribution maybe achieved at the electron emitting surface even when heat is input to the main cathode unevenly by setting the thickness t of the main cathode relatively large to satisfy the following expression (see Electron Ion Beam Handbook 2.2.1 (2), p23, 24).
t greater than Qa2/5kT
It is to be noted that in the above expression, a represents the radius of the main cathode, Q represents the density of the quantity of heat input to the main cathode, k represents the heat conductivity of the main cathode material and T represents the cathode operating temperature.
The dimensions of an illumination beam on a reticle in an electron beam exposure apparatus adopting a divided-pattern transfer method, the use of which is currently being considered for application in the exposure process implemented to manufacture next-generation semiconductor devices such as 4G DRAMs and the like, may be, for instance, 1 mmxc3x971 mm. The diameter of the electron emitting surface of an electron gun that may be employed in such an electron beam exposure apparatus is 8 mm, and the electron gun achieves a beam current of 100 xcexcA at the reticle and a 2 mm mrad emittance.
However, there is a problem with the electron gun in the prior art having a flat cathode in that the intensity of the electron beam is lowered at the circumferential area of the main cathode.
In addition, while the filament at the sub-cathode in the known art is often constituted by concentrically coiling two lines that are aligned with each other, such a filament structure does not achieve a sufficient degree of non-inductive characteristics, and thus, the magnetic field attributable to the current flowing through the filament may adversely affect the electron beam emitted from the main cathode.
An object of the present invention is to provide an electron gun capable of emitting a large beam achieving a more uniform intensity distribution.
Another object of the present invention is to provide an electron gun capable of minimizing the degree to which an electron beam emitted from the main cathode becomes bent.
Yet another object of the present invention is to provide an electron beam exposure apparatus capable of performing exposure by using a large beam achieving a more uniform intensity distribution.
In order to achieve the objects described above, the electron gun according to the present invention comprises a main cathode having an electron emitting surface at one surface of a plate member and a sub-cathode facing opposite another surface of the main cathode, with a filament coiled to achieve a double helix structure constituting the sub-cathode. By utilizing the filament coiled in a double helix structure as described above, the magnetic fields caused by the currents flowing through the adjacent filaments can be canceled out by each other, to ultimately minimize the degree to which the electron beam emitted from the main cathode becomes bent.
In addition, by setting the temperature at the outer circumferential area of the electron emitting surface of the main cathode higher than the temperature at the remaining area of the electron emitting surface, an electron beam with a uniform intensity distribution is obtained. For instance, by allowing the sub-cathode to expand further outward relative to the outer circumferential area of the main cathode and thus setting the area of heat application surface of the sub-cathode larger than the area of the main cathode, the temperature at the outer circumferential area of the electron emitting surface can be set higher than the temperature at the center.
Furthermore, an electron beam achieving a uniform intensity distribution can be obtained by constituting the sub-cathode of the electron gun with a filament coiled in a double helix structure and setting the ratio of the external diameter of the double helix and the diameter of the main cathode so as to optimize the density of the electrons flowing from the sub-cathode to the main cathode or the cathode radius directional dependency of the temperature at the electron emitting surface of the main cathode.
At the same time, by setting the thickness t of the main cathode so as to satisfy the following expression, a temperature distribution that better reflects the current density with regard to the electron bombardment at the rear surface of the main cathode can be achieved with ease to ultimately realize a better temperature distribution along the radial direction of the electron emitting surface.
t less than 5Qa2/kT
with a representing the radius of the main cathode, Q representing the density of the quantity of heat input to the main cathode, k representing the heat conductivity of the main cathode material and T representing the cathode operating temperature.
Also, by utilizing the electron gun described above in an electron beam exposure apparatus, highly accurate exposure can be implemented over a relatively large exposure area to improve the throughput of the exposure process.
Moreover, by employing the electron beam exposure apparatus in semiconductor device production, it becomes possible to manufacture a semiconductor device achieving a smaller line width and a higher degree of integration.